[LU-12373] detach and delete the PCC cached files when remove a PCC backend from a client

• Type: Technical task
• Resolution: Unresolved
• Priority: Minor
• Assignee: Qian Yingjin
• Reporter: Qian Yingjin

Description

Currently when remove a PCC backend from a client, it does not deal with previously PCC-cached files at all. These files can still use PCC caching service.

That's to say, after remove the corresponding PCC backend:

1) For RW-PCC cached files, we do not restore the data back into Lustre OSTs of the main filesystem. Although the PCC backend falls back as a tranditional HSM storage solution since the lhsmtool_posix copytool is still running at this client. But this is dangerous, and likly to cuase user data to be lost if the PCC device maybe permanently unaviailable.

2) The space used by these PCC cached files may not release.

 

Thus, we need to add an option to detach and delete the PCC cached files when remove a PCC backend from a client.

The practical and feasible methods is to scan the PCC backend fs, execute the detach operations via "lfs pcc detach" command or the corresponding llapi.

i.e.  For RW-PCC cached files, it can do as follows:

 "find /mnt/pcc -type f -name '[0-9a-f:]'" to generate a FID list, then "lfs pcc detach_fid $fid" for each FID.

 

Here we still need to patch the PCC detach, as the the scanned regular files in the PCC backend fs may be detached involuntary by the Layout lock revocation due to the LRU lock shrinking or by the inode reclaim. 

For these kind of files,  PCC detach still needs to restore the file into Lustre OSTs, wait the restore finish, and then send HSM remove requests to remove them for RW-PCC; while for RO-PCC, we need to simply delete the crresponding files.